1. Field of the Invention
This invention relates to an electrostatic printing method utilizing electrostatic copying methods where liquid ink is applied from a developing surface having substantially uniform projections thereon to develop an electrostatic latent image, the spaces between the projections being filled with the ink.
2. Discussion of the Prior Art
The term electrostatic copying method as used throughout this specification is directed to such processes as (1) an electrophotographic copying method where a photoconductive surface is charged with electricity, and exposed to a light image, to thereby form an electrostatic latent image thereon; (2) an electrostatic recording method where a dielectric surface is scanned with a needle electrode to form an electrostatic latent image thereon; or (3) a copying method where a photoconductive member is impressed with bias voltage and then exposed to a light-image to form an electrostatic latent image thereon. In these electrostatic copying methods, developing methods are known such as the cascade developing method and the liquid developing method, which is characterized by electrophoresis. The liquid developing method is advantageous in that the dimensions of the developing unit are reduced and image resolution is high. The liquid developing method, however, has drawbacks in that a latent image bearing photoconductive sheet must be immersed in its entirety into the liquid developer and thus the background area tends to be stained.
To reduce the possibility of background area stain to a minimum, the amount of toner contained in the carrier insulating liquid of the developer must be controlled, its permissible range being within 0.5% at the most. Accordingly, when a developed picture image of high density is desired, the liquid developing method is not available.
In order to overcome the above-described drawbacks, an electrostatic attraction developing method, as disclosed in the specifications of U.S. Pat. No. 3,084,043 and Japanese Patent Publication No. 44-9,512(1970), has been utilized where a liquid ink is applied to a latent image bearing member having an electrostatic charge pattern or a conductive pattern to effect a wet development. An important feature of this developing method is that the developer ink is applied from a geometrically uneven developing surface. More concretely, the liquid ink applying member may comprise an iron pipe having fine spiral grooves whose pitch may be as small as about 0.1 mm. When developing, liquid ink fills the grooves with excess ink being removed therefrom by a scraper means such as a doctor blade. The ink applying member is brought into contact with an electrostatic latent image bearing photoconductive material so that the liquid ink adjacent the electrostatic latent image is attracted by an electrostatic attracting force whereby the ink is attracted to the crest portions of the afore-said grooves to moisten the photoconductive material and thereby effect the desired development.
In the afore-said electrostatic attraction developing method, a roll-shaped developer applying member is typically used and unlike the first-mentioned liquid developing method, the photoconductive sheet need not be dipped in the developing liquid. Accordingly, only a small amount of developing liquid is used. This advantageously permits further miniaturization of the developing unit, as compared with that of the first-mentioned liquid developing method. In the developing method where ink is attracted to an area corresponding only to the latent image formed on the photoconductive material, there is less limitation on the toner density. Thus, a toner of high density may be used unlike the first-mentioned liquid developing method.
Thus, the aforesaid electrostatic attraction developing method is superior in many aspects to the liquid developing method. The former method, however, still presents the following difficulties. Assume an electrostatic latent image exists adjacent the grooves of an ink applying member b, as shown in FIG. 1. The bank areas and their immediate vicinities alone are moistened with liquid ink as shown by reference numeral c in FIG. 2, with an inter-area between bank areas remaining unmoistened, as shown in FIG. 3. Although this phenomenon hardly occurs where the charge density of the electrostatic latent image is high, it is very often experienced in areas where electric charge density, such as accompanies an image of neutral tone, is low. For this reason, those portions of the latent image adjacent a trough portion of the grooves provided on ink applying member b, may not be developed as shown at d. This lowers the density of developed image, causes discontinuation in lines, and skips in a picture image, all of which degradate image quality.